1. Intact frog single fibres were investigated under control conditions (1 s tetanus every 2, 3 or 5 min) and during moderate fatigue (interval between tetani 15 or 30 s). 2. Fatigue reduced isometric force (P0) by 25.8 +/- 1.6% (S.E.M.; n = 13) and depressed the maximum velocity of shortening (Vmax) by 10.2 +/- 2.2% (n = 13). The force-velocity relation became less curved, a/P0* (see Methods) being increased by 29.5 +/- 8.8% (n = 13). Thus, power was less affected than isometric force or Vmax. 3. The velocity of unloaded shortening (V0), from slack test measurements, was reduced proportionally more than Vmax during fatigue. Under control conditions V0 was larger than Vmax, but during fatigue their values were not significantly different. 4. Stiffness during shortening was reduced during fatigue indicating fewer attached cross-bridges in fatigue. Force was reduced more than stiffness indicating that, on average, there is less force per attached cross-bridge. 5. The force-lengthening velocity relation showed that the ability to resist forces greater than isometric was well preserved in fatigue. 6. Compared with fatigue, intracellular acidification with CO2 produced a smaller reduction in isometric force. However, reduction in Vmax was not significantly different from that in fatigue. These results are consistent with both inorganic phosphate and H+ increasing in fatigue, but only H+ increasing during acidification, and isometric force being reduced by both, Vmax being sensitive only to H+.